And one out of every six people will have one at some point in their lives.

Strokes deprive brain cells of oxygen and are one of the most common causes of death and a leading cause of preventable disability.

When someone experiences a stroke, quick medical care is critical , and can often help avoid permanent brain damage.

But what causes strokes in the first place?

And what can doctors do to treat them?

The brain makes up just 2% of your body's mass but consumes more than 20% of the oxygen in your blood.

That oxygen is carried to the brain through a system of arteries.

Carotid arteries supply the front of the brain, and vertebral arteries supply the back.

These are connected to each other, and divide into smaller and smaller vessels that get billions of neurons the oxygen they need.

If the blood flow is interrupted, oxygen delivery stops and brain cells die.

There are two ways this can happen.

Hemorrhagic strokes are when a perforated vessel allows blood to leak out.

But the more common type is the ischemic stroke, when a clot blocks a vessel and brings blood flow to a halt.

Where do these clots come from?

On rare occasions, a sudden change in heart rhythm prevents the upper chambers of the heart from contracting normally.

This slows down blood flow, allowing platelets, clotting factors, and fibrin to stick together.

The clot can be carried up towards the arteries and blood vessels supplying the brain until it gets to one it can't squeeze through.

This is called an embolism and it cuts off the oxygen supply to all the cells downstream.

The brain doesn't have pain receptors, so you can't feel the blockage itself.

But oxygen deprivation slows brain function and can have sudden, noticeable effects.

For example, if the affected area is responsible for speech, an individual's words may be slurred.

If the stroke affects a part of the brain that controls muscle movement, it can cause weakness, often just on one side of the body.

When this happens, the body will immediately try to compensate by diverting blood flow to the affected area, but this isn't a perfect solution.

Eventually, the oxygen-deprived cells will start to die, leading to brain damage that may be severe or permanent.

That's why it's important to get medical care as fast as possible.

The first line of treatment is an intravenous medication called Tissue Plasminogen Activator, which can break up the blood clot and allow blood to flow again in the compromised artery.

If it's delivered within a few hours, this medication greatly increases the chance of surviving the stroke and avoiding permanent consequences.

If Tissue Plasminogen Activator cannot be given because the patient is on certain medications, has history of major bleeding, or the clot is particularly large, doctors can perform a procedure called an endovascular thrombectomy.

Using a fluorescent dye that illuminates the blood vessels under a strong x-ray, the physician inserts a long, thin, flexible tube called a catheter into an artery in the leg and maneuvers it all the way to the blockage.

A retriever is passed through this catheter.

It expands and anchors into the clot when it's just past it.

The catheter then pulls the clot out when it's removed.

These treatments need to be delivered as soon as possible

to preserve brain function,

which means figuring out fast if someone is having a stroke.

So how can you tell?

Here are three quick things to try:

1. Ask the person to smile.

A crooked mouth or facial drooping can indicate muscle weakness.

2. Ask them to raise their arms.

If one drifts downward, that arm weakness is also a sign of a stroke.

3. Ask them to repeat a simple word or phrase.

If their speech sounds slurred or strange,

it could mean that the language area of their brain is oxygen-deprived.

This is sometimes called the FAST test, and the T stands for time.

If you see any of those signs, call emergency services right away.

Lives may depend on it.

We are on the non-profit mission to revolutionize education. Click here to learn how you can get involved.